1. Field of the Invention
This invention relates to a digital signal transmission system for effecting time-division multiplexing/circuit switching transmission of data in the form of packets by means of a telecommunication cable. More particularly, this invention relates to a digital signal transmission system which is improved in terms of connection control characters to be contained in the overhead fields of packets. The present invention especially relates to a digital signal transmission system which is improved in terms of configuration of packets in the data transmission phase.
2. Description of the Prior Art
The dissemination of electronic computers and the growth of digital signal processing techniques have recently culminated in successful combination of telecommunication systems with data processing systems and perfection of data communications devoted to on-line data processing. These achievements are attracting mounting public interest and respect.
Particularly in the small-scale communication systems such as for the intraorganizational communication confined within the premises of a government or public office or of a private corporation, the system which effects communication in the form of packets by means of a telecommunication cable such as a coaxial cable is arresting particularly keen interest owing to its features such as good economy, high reliability, and prominent efficiency of transmission.
This communication system which makes use of packets requires installation, as in laboratories, of communication cables adapted to effect transmission in both directions and connection to these communication cables of numerous personal stations. From these personal stations, messages divided into data blocks each of 1000 to 2000 bits, for example, are transmitted through the telecommunication cables. The individual messages are each prefixed with a header covering such information as address and serial number.
In the communication system of this nature, the network itself is a passive transmission medium totally devoid of any control function and the individual personal stations have such control functions thoroughly distributed among themselves.
At a given personal station, therefore, transmission of a messabe is started when an idle channel is available in the cables. When a packet of message transmitted from one personal station collides with a packet of message transmitted from another personal station, these two personal stations discontinue the transmission of their messages. The personal station which has discontinued the transmission, on elapse of a random queuing time, tries to resume the transmission of the message.
In the communication system of this operating principle, users posted near the personal stations enjoy access to a common computer to which the communication system is interfaced. They are also able to have common use, through the medium of this communication system, of various items of hardware such as memory devices and various items of software such as programs distributed throughout the organization.
The devices such as high-speed or high-precision printers and large-capacity files which are concentrated at the large-scale central processing unit in the time-sharing system can be used in th communication system as widely distributed within the premises of a given organization.
The communication system, therefore, enables the user to enjoy economization of resources and improvement in efficiency of utility. Besides, it warrants ample flexibility of programs and data and promises development of an intensive software system.
Further, the communication system of this nature has an advantage that all the personal stations are equally entitled to the use of channels, namely no special personal station has priority on the use of a channel over the remainders. This means that the present communication system does not have the hierarchical relationship often found among the personal stations in communication systems of other operating principles, so that communication can be established between two freely connected personal stations.
It has another advantage that the system can be easily designed in a highly reliable network because the component channels such as of coaxial cables are invariably formed of completely passive circuits.
One version of the digital signal transmission system of the operating principle described above contemplates causing frames which are periodically repeated on the time base to be each divided into a plurality blocks similarly on the time base and enabling the individual personal stations to find chances to effect packet communication by the unit of such blocks (as proposed by Japanese Patent Application No. SHO 56(1981)-38714, for example).
In this signal transmission system, all the personal stations are equally entitled to the use of idle blocks. In case where a given personal station occupies a specific block over a duration necessary for signal transmission, that personal station is periodically given a chance for signal transmission in each of frames repeated on the time base. Thus, this system permits the personal stations to effect real-time transmission of signals by making use of the function described above.
One typical frame configuration for the signals to be used in the aforementioned digital signal transmission system is illustrated in FIG. 1.
Each of the frames which are repeated periodically on the time base consists of N blocks (#1 through #N). And each of the blocks consists of various bit rows, b.sub.1 through b.sub.9, as shown below.
b.sub.1 . . . Backward guard time PA1 b.sub.2 . . . Preamble PA1 b.sub.3 . . . Start flag PA1 b.sub.4 . . . Address bit PA1 b.sub.5 . . . Control bit PA1 b.sub.6 . . . Data bit PA1 b.sub.7 . . . Check bit PA1 b.sub.8 . . . End flag PA1 b.sub.9 . . . Forward guard time PA1 A. Establishment of call PA1 B. Transmission of data (communication) PA1 C. Termination of call PA1 (1) In the phase of the establishment of a call, the called station goes over the periodically repeated frame and seeks out the particular block containing the packet addressed to itself by reference to the block number assigned to the block, for example. PA1 (2) In the phase of the transmission of data (communication) after completion of the establishment of a call, the station takes in the selected packet on the basis of the positional data of the packet (block number). PA1 (3) Owing to the adoption of the measures indicated in (1) and (2) above, the capacity for data transfer per packet is increased and the efficiency of transfer is improved by decreasing to the irreducible minimum of requirement the overhead field to be used for the so-called connection control in the packet transmitted during the phase of the transmission of data and inserting additional data bits in the room consequently vacated in the overhead field.
The bit rows b.sub.2 through b.sub.5 and the bit rows b.sub.7 and b.sub.8 are essential components for a packet. These bit rows are collectively referred to as "overhead bits." The two bit rows, b.sub.1 and b.sub.9, are collectively referred to as the "guard time."
The term "guard time" means "empty bit rows" which are intended to preclude the situation in which packets in adjacent blocks may possibly be caused to overlap, if partially, owing to the delay time which occurs during the propagation of signals on a coaxial cable.
In the bit rows forming this guard time, the backward guard time b.sub.1 serves to protect the trailing one of any two adjacent packets against the trouble of overlapping and the forward guard time b.sub.9 similarly to protect the leading packet against the trouble.
FIG. 2 illustrates in outline a typical communication system which suits the transmission of digital signals of the frame configuration described above.
In this communication system, a coaxial cable 3 laid as a channel has its opposite ends connected to impedance-matching terminators 1, 2 having electrical resistance equal in magnitude to characteristic impedance of the said cable.
The personal stations are connected through the respective T connectors (taps), 4.sub.1 -4.sub.N, to the coaxial cable 3. All these personal stations have a basically identical structure. To avoid wasteful repetition, therefore, the essential components of the personal station, S, connected through the T connector 4.sub.1 are illustrated representatively in the diagram.
Each of the personal stations is provided with a user equipment 5 incorporating a computer and a telephone.
The user equipment 5 is further provided with a transmitter (encoder) 51 for transmitting digital signals of the unit of packets to some other personal station, a receiver (decoder) 52 for receiving digital signals of the unit of packets transmitted by some other personal station, and a terminal control unit 53 for controlling terminals.
The signals issued from the transmitter 51 are temporarily stored in a transmission buffer memory 61. The stored outgoing signals are then read out of the buffer memory 61 at a prescribed time by clock signals equalling the transmission speed on the coaxial cable 3 serving as a transmission medium. The signals thus read out are converted into packets of a prescribed pattern by a transmit logical circuit 62. These packets are sent through a transmit buffer amplifier 63 and forwarded via the T connector 4.sub.1 onto the coaxial cable 3.
All the packet signals which are being transmitted on the coaxial cable 3 are sent through the T connector 4.sub.1, received by a receive buffer amplifier 64, and supplied to a receive logical circuit 65.
The receive logical circuit 65 selects, out of all the incoming packets, only the packets addressed to its own personal station and puts the selected packets to temporary storage in a receive buffer memory 66. The packet signals thus kept in storage are read out continuously by the use of a prescribed clock in the receiver 52. Thus are obtained received output signals.
The transmission and reception of signals are accomplished as described above. The transmission clock which is used in this case is generated by a clock generator 67. A frame counter 68 serves to divide this transmission clock to produce a block timing signal 72 and a frame timing signal 73.
A transmission control unit 69 effects control of the terminal control unit 53 in accordance with reception signals addressed to its own personal station and derived from the receive logical circuit 65 and, at the same time, controls the transmit logical circuit 62 in accordance with instructions from the terminal control unit 53.
A collision detector 74 serves, at the time that the first packet signal is issued on the block selected by its own personal station, to check and confirm whether or not that packet signal has collided with a packet signal issued from some other personal station.
In the present digital signal transmission system, no transmission of any signal occurs on the coaxial cable 3 while none of the personal stations is engaging in the transmission or reception of signals. So far as this state lasts, signals being the reference of the frames and signals indicative of the positions of division of the individual blocks are not transmitted on the coaxial cable 3.
In this system, therefore, the initiative in both frame synchronization and block synchronization is awarded to the particular personal station that is the first to effect transmission of signals. While the first personal station is transmitting signals with block length and frame cycle agree upon with the other personal station, some other personal station urged to use the communication system is required to establish proper block and frame timings on the bases of the packets issued from the first personal station, select empty blocks of its own accord, and occupy these blocks for the transmission of signals exclusively for a required duration.
As soon as the first personal station completes its communication while some other personal station is still transmitting its own signals, such other personal station automatically assumes the initiative in frame synchronization, and the like.
Generally, the circuit switching operation involved in the digital signal transmission system of the operating principle described above may be divided into the following three phases.
Now, these phases the circuite switching operation will be described below with respect to the aforementioned digital signal transmission system.